Ambient vibrations recording

Ambient vibrations were measured with two sets of sensors: 10 three-component Lennartz sensors (resonance frequency of 0.2 Hz) and 22 vertical 4.5 Hz geophones used for the $P-S_V$ experiments. For the first array (A), 22 4.5 Hz vertical sensors were set up 5 m apart on a 17.5 m radius circle (figure 6.27(a)). The hole in the circle of figure 6.27(a) comes from a defective receiver, resulting in 21 available signals. The ten Lennartz sensors were set up with three distinct geometries (Arrays B to D, shown on figures 6.27(b) to 6.27(d), respectively). The geometry of arrays B and D was made of one central sensor and three triangles rotated by 40$^{\circ}$ , with maximum apertures of 40 and 50 m, respectively. Array C has a central sensor and nine sensors distributed on a circle with a radius of 40 m. All the sensor positions were measured with a theodolite, expecting a centimetric accuracy.

Figure 6.27: Local maps showing the array geometries (black dots), the refraction lines (shots with grey stars and receivers with grey circles), borehole and cone penetration test locations (grey square and grey squares with cross, respectively). Y axis is oriented towards the magnetic North (site of the old Bavière Hospital, Liège, Belgium, 50.64$^{\circ}$ N, 5.57$^{\circ}$ E, 19th March 2002). Each ambient noise array geometry is represented on a separate plot: (a) Array A, (b) Array B, (c) Array C and (d) Array D.

Figure 6.28: Spectral curves of the central station of array A, B and C. The plain line is the average and the dashed lines are located at one standard deviation. (a) Amplitude spectrum of the vertical component. (b) Amplitude spectrum of one horizontal component. (c) Spectral ratio Horizontal to Vertical (H/V). Grey bands indicate the average and standard deviation of the frequency peak values observed for each individual time window.

The horizontal to vertical spectra are calculated for all individual three-component signals. Typical spectra are shown for the central station in figures 6.28(a) to 6.28(c). The plain line is the average of all time windows while the dashed lines are drawn at one standard deviation (geometrical average). The total recording length is 6 hours. Statistics are calculated over 3690 time windows of five seconds each. The vertical and an horizontal component spectra are presented in figures 6.28(a) and 6.28(b), respectively. A clear high pass filter effect is observed for the vertical component as demonstrated by Scherbaum et al. (2003) for a synthetic case. The ratio of the vertical to the horizontal average component is shown in figure 6.28(c) and it exhibits a well developed peak at 5.3 Hz. The results are quite similar for the other stations, with a mean frequency varying from 4.8 to 5.3 Hz with a standard deviation of about 0.5 Hz. The average value of the peak frequency is 5.17$\pm$ 0.57 Hz (29015 time windows of five seconds) over the whole area. The small spatial variation of the resonance frequency is supporting the assumption of a one-dimensional structure.